In order to meet a high demand for miniaturization and weight saving of various radiocommunication instruments, it is necessary to miniaturize an antenna mounted thereon.
For example, a portable telephone resides in a technical field particularly high in market demand for miniaturization and weight saving because it is a portable information communication device. The portable telephone has labored in housing the whole device in a small housing (case) from the beginning of its development. Recently, more miniaturization has been required because it is necessary to also mount antennas for GPS (global positioning system) and information communications different in frequency in addition to the antenna for telephone. More specifically, the portable telephone tends to increase the kinds of information taken, and frequency bands used are different from one another according to the kinds of information taken, for example, telephone (0.8 GHz or 2 GHz band), GPS (1.5 GHz band) and information communications (Bluetooth and the like, 2.4 GHz band). Therefore, plural antennas meeting these kinds of information are required, and so it is necessary to more miniaturize these plural antennas for housing the antennas in a case.
There are, for example, an antenna for GPS and an antenna for automatic fare collection system as automobile antennas, and these antennas are desirably miniaturized because there is a possibility that the field of vision may be obstructed, or the antenna may come into contact with a vehicle occupant when installed in a vehicle.
A great number of radiocommunication instruments such as a TV, a radio, a GPS, an automatic fare collection system terminal, a portable telephone and an inter-vehicle distance sensor is installed in a motorcar. The frequency bands used for these radiocommunication instruments are different from one another. Therefore, it is necessary to miniaturize each antenna element for installing plural antennas for the respective communication information instruments different in frequency bands used.
A personal computer (PC), in which antennas for information communications such as wireless LAN, Wi-Fi and further WiMax, which is a next-generation communication, are installed, has been developed. The antennas used for these information communications are also required to be miniaturize and lightened for housing them in a case with a limited space.
A dielectric antenna easy in production, and capable of controlling the dimensions of a circuit pattern with high precision and lessening a scatter of performance is often used as an application antenna mass-produced for portable telephones, motorcars, personal computers, etc. The dielectric antenna has, for example, a structure that a copper foil is laminated on a substrate formed of a dielectric, and a circuit pattern is formed on the copper foil.
In order to miniaturize the dielectric antenna, it is necessary to raise the relative dielectric constant of the dielectric antenna. When the relative dielectric constant of the dielectric antenna is raised, the wavelength of a propagating signal becomes short, and so the miniaturization is allowed. However, a conventional dielectric material shows a tendency for its dielectric loss tangent to also rise with the rise of the relative dielectric constant. Energy loss becomes small as the dielectric loss tangent of the dielectric antenna is small, and so the performance of the antenna is improved. When the relative dielectric constant of the dielectric antenna is raised to miniaturize it, the dielectric loss tangent thereof becomes high to lower the performance of the antenna.
Japanese Patent No. 3664094 (Patent Literature 1) has heretofore disclosed a composite dielectric molded product obtained by molding a composite dielectric material containing a dielectric inorganic filler and an organic polymeric material and proposed the use of the composite dielectric molded product as a lens antenna. However, the relative dielectric constants of the composite dielectric molded products specifically shown in the respective Examples of Patent Literature 1 are relatively low.
Japanese Patent No. 3895175 (Patent Literature 2) has proposed a dielectric resin integrated antenna equipped with a dielectric formed of a dielectric resin composite material with dielectric ceramic powder having a dielectric constant (relative dielectric constant) of 20 or more and a dielectric loss tangent of 0.005 or less incorporated therein. However, the relative dielectric constants of the dielectric resin composite materials containing the ceramic powder specifically shown in the respective Examples of Patent Literature 2 are not very high, and on the other hand, the lower limit of dielectric loss tangents thereof is relatively high.
Japanese Patent Application Laid-Open No. 2005-94068 (Patent Literature 3) discloses a composite material with dielectric ceramic powder contained in a synthetic resin. The composite material shows a relative dielectric constant of 15 or more and a dielectric loss tangent of 0.01 or less in a frequency range of 100 MHz or more. However, the dielectric loss tangents of the composite materials specifically shown in the respective Examples of Patent Literature 3 are about 0.003 to 0.005 and are thus not sufficiently high though the relative dielectric constants thereof are as high as about 17 to 52 as shown in Paragraph [0023] of the description. In other words, the dielectric loss tangent of the composite material could not be reduced to the degree lower than 0.003 in the state of the art described in Patent Literature 3.
International Publication No. 2008/081773 Pamphlet (Patent Literature 4) discloses a flame-retardant resin composition containing polybutylene terephthalate, a bromine-containing flame retardant, an antimony-containing flame retardant aid and a titanic inorganic compound. The flame-retardant resin composition is used as an antenna part. However, with respect to the dielectric characteristics of the respective flame-retardant resin compositions shown in the respective Examples of Patent Literature 4, the relative dielectric constants are relatively low, and on the other hand, the lower limit of dielectric loss tangents thereof is 0.008 and relatively high.
As described above, a dielectric antenna using a dielectric material low in dielectric loss tangent while having a high relative dielectric constant, i.e., a dielectric antenna having high performance and sufficiently miniaturized has been unable to be provided according to the prior art. Therefore, there is a demand for developing an antenna substrate high in relative dielectric constant and sufficiently low in dielectric loss tangent for providing a dielectric antenna miniaturized and having high performance.